JPH0613707A - Semiconductor laser - Google Patents

Abstract

PURPOSE:To form a longitudinal multimode oscillation at low output, and to lower the noise level in an a GaAs self-matching type MQW structure semiconductor laser having a complex refractive index waveguide mechanism by specifying the film thickness of a current-limiting layer. CONSTITUTION:A clad layer 3, an MQW active layer 4, a lower clad layer 5 and a current-limiting layer 6 are laminated successively on an N-type GaAs substrate by a molecular beam epitaxial method. Then, a stripe groove is formed to the depth wherein the prescribed thickness of the GaAs of the current-limiting layer is left by conducting an etching treatment. Then, after the lower clad layer 5 has been exposed, with an arsenic molecular beam applied while the substrate is heated up, an upper clad layer 7 and a cap layer 8 are laminated. After upper and lower electrode films 1 and 9 have been formed, the substrate is separated into chips. In this case, the film thickness of the current-limiting layer 6 is set at 2000 to 6000 angstrom, optically induced noise is sharply reduced, and a longitudinal multimode oscillation is accomplished.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a semiconductor laser.
More specifically, it is used as a light source for pickups of compact discs and magneto-optical discs, and is capable of a single transverse mode up to high-power operation, and is a self-aligned high-frequency oscillator that has multi-longitudinal mode oscillation at low-power operation. The present invention relates to an output semiconductor laser.

[0002]

2. Description of the Related Art Recently, there has been a strong demand for a high-power semiconductor laser having a light output of 30 mW or more as a light source for disk writing of a magneto-optical disk.

Characteristics required for a semiconductor laser for a magneto-optical disk are (1) single transverse mode oscillation up to high power operation, (2) low astigmatic difference, (3) low ellipticity, (4) Low noise and (5) high reliability.

Among these, in order to obtain the single transverse mode oscillation up to the high output operation (1), in the self-aligned MQW structure semiconductor laser having the complex index guiding mechanism, for example, the active layer is thin. And a film thickness design such as shortening the distance between the MQW active layer and the current limiting layer to provide a high-refractive-index waveguide type structure (Ryoichi Ito et al. “Semiconductor laser”, Baifukan (1989)).

[0005]

However, when a high refractive index waveguide type structure is used, a low output operation (for example, an optical output of 3 m) is obtained.
Also in W), it becomes easy to oscillate in a single longitudinal mode and the coherence becomes high. It is known that such a semiconductor laser is apt to generate return light induced noise.
The low noise of (4) cannot be satisfied.

[0006] As a conventional countermeasure, it has been attempted to reduce noise by creating a multi-longitudinal mode by superimposing a high frequency (see "Electronics of D. Welford" and "A. Moradian"). Observation of linewidth broadening in (GaAl) As diode lasers due to number fluctuations
(GaAl) As diode lasers due to electron number fluct
uations) (Applied Physics Letter (Appl.
Phys. Lett.), Vol. 40, p. 560, 1982)), and it was difficult to sufficiently reduce the noise level even when the high frequency was superposed if the longitudinal mode unity before the high frequency superposition was strong.

On the other hand, as a means for making a multi-longitudinal mode,
A method using a gain-guided structure is known (Ryoichi Ito et al. “Semiconductor laser”, Baifukan (1989)), but in the gain-guided structure, it is necessary to maintain single transverse mode oscillation up to high output operation. However, there is a problem that the astigmatic difference increases.

As a result of intensive studies made by the present inventors in view of the above circumstances, when the thickness of the current limiting layer was reduced from 8000 to 10000 Å in the past, when the film thickness became 6000 Å or less, a single lateral The present invention has been completed by discovering that a multi-longitudinal mode is started while maintaining the mode. That is, an object of the present invention is to provide a complex refractive index waveguide which is a multi-longitudinal mode oscillation in a low output operation (for example, an optical output of 3 mW) while having a refractive index waveguide type structure capable of a single transverse mode up to a high output operation. A self-aligned MQW structure high-power semiconductor laser having a mechanism is provided.

[0009]

The semiconductor laser of the present invention is an AlGaAs self-aligned MQW structure semiconductor laser having a complex refractive index guiding mechanism, in which the thickness of the current limiting layer is 6000 to 2000 Å or less. It is characterized by being.

Another aspect of the semiconductor laser of the present invention is characterized in that the distance between the MQW active layer and the current limiting layer is 2500 to 3500Å.

[0011]

According to the present invention, the thickness of the current limiting layer is 6000-20.
Since it is set to 00Å, the longitudinal mode oscillation at low output is multi-valued and the noise level can be sufficiently reduced.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the semiconductor laser of the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 (a) is a sectional explanatory view of an embodiment of a self-aligned MQW structure semiconductor laser of the present invention. Figure 1
In (a), 1 is an N-side electrode, 2 is an N-type GaAs substrate,
3 is N-type Al _0.6 Ga _0.4 As cladding layer, 4 is N-type A
In the MQW active layer of _{l y Ga 1-y As,} Al 0.22 Ga 0.78
As barrier layers and GaAs well layers are alternately laminated to form a multiple quantum well structure as shown in the energy band structure in FIG. 1 (b). 5 is P-type Al _0.6 Ga _0.4 As
Lower cladding layer, 6 N-type GaAs current limiting layer, 7 P-type Al _0.6 Ga _0.4 As upper cladding layer, 8 P-type GaAs
The cap layer and 9 are P-side electrodes, respectively.

To manufacture a semiconductor laser having this structure,
First, by the first MBE (Molecular Beam Epitaxy) method, the N-type Al _0.6 Ga _0.4 As layer is used as the cladding layer 3 and the N-type Al _0.22 Ga _0.78 As is used as the MQW active layer 4 on the N-type GaAs substrate 2. Layers of 68 Å and GaAs layers of 40 Å alternately laminated, P-type Al as the lower clad layer 5
_0.6 Ga _0.4 As and N-type Ga as the current limiting layer 6
Each layer of the As layer (these are referred to as a first growth layer) is sequentially laminated by changing the molecular beam strengths of the crystal growth material and the dopant material. MQW active layer is A while growing GaAs.
1 The shutter is formed by opening and closing in a short time. As N-type dopant metal, use Si or Sn,
Be is used when P type is used. Next, the current limiting layer 6 is removed from the surface of the first growth layer by an etching process.
Dig the stripe groove to the depth where about 1000 Å of GaAs remains.

The substrate in which the stripe grooves have been dug in this way is again placed in the MBE apparatus, and the surface of the substrate is first irradiated with an arsenic molecular beam while heating the substrate to evaporate GaAs at the bottom of the stripe grooves and to lower the cladding layer. After exposing
P as the upper cladding layer 7 by the second MBE process
Type Al _0.6 Ga _0.4 As and P as the cap layer 8.
Type GaAs layers are stacked. Finally, after the upper and lower electrode films 1 and 9 are formed, the substrate is cut to separate each chip.

A feature of the present invention is that the film thickness of the current limiting layer 6 is specified. When the film thickness is 6000 to 2000Å, more preferably 3000 to 5000Å, a single transverse mode oscillation up to high output operation is achieved. A self-aligned high-power semiconductor laser capable of multi-longitudinal mode oscillation in a low-power operation (for example, a light output of 3 mW) can be obtained in spite of its refractive index guided structure.

That is, the thickness of the current limiting layer 6 was variously changed, and the change of the relative intensity noise (RIN) in the presence of returning light was examined.

The thickness of the current limiting layer 6 can be changed easily by controlling the time when the GaAs layer is grown by the above-mentioned MBE apparatus, for example, Ga.
When the growth rate of As is 1.0 μm / h, a crystal growth of about 60 minutes is required to form a thickness of 10,000 Å, and a crystal growth of about 36 minutes is required to form a thickness of 6000 Å. To be By this method, the thickness (unit: Å) of the current limiting layer 6 is set to 10
000, 7000, 6000, 4500, 3500, and 2000 were manufactured, and the change in relative intensity noise was examined for each of them when P _o = 3 mW, f _c = 1 MHz, and the amount of returning light was 1%.

The results are shown in FIG. As can be seen from FIG. 2, the return light induced noise becomes smaller as the thickness of the current limiting layer 6 becomes thinner.

From this result, the thickness of the current limiting layer was 6000Å
It was found that the return light induced noise is significantly reduced and multi-longitudinal mode oscillation can be obtained by the following. Also, from the viewpoint of multi-longitudinal mode oscillation, the thinner the current limiting layer 6 is, the more preferable data is obtained.
The function as the current limiting layer 6 cannot be sufficiently exhibited, the current region is widened, and the luminous efficiency is reduced, which is not preferable. Therefore, it is preferable to set the thickness to 6000 to 2000Å. Further, in order to further reduce the noise and improve the luminous efficiency, it is more preferable to set it to 5000 to 3000Å.

Further, in order to obtain the stability of the transverse mode and the low astigmatic difference, the distance between the active layer and the current limiting layer (the film thickness of the p-type lower cladding layer) is preferably 2500 to 3500Å. In any case, by setting the thickness of the current limiting layer of the present invention to 6000 to 2000Å, the multi-longitudinal mode in low output operation can be realized. Therefore, the distance between the MQW active layer and the current limiting layer
By setting the thickness of the current limiting layer to 2500 to 3500Å and 6000 to 2000Å, it is possible to satisfy the stability of the transverse mode at high output and low astigmatic difference, and to achieve the multi-longitudinal mode at low output. , The noise level can be reduced sufficiently.

Next, FIG. 3 shows the result of measuring the characteristics of the longitudinal oscillation mode at an optical output of 3 mW which is a low output of one embodiment of the semiconductor laser of the present invention in comparison with the characteristics of the semiconductor laser of the conventional structure. 3A shows the characteristics of the semiconductor laser according to this embodiment, and FIG. 3B shows the structure of the conventional semiconductor laser for comparison. In the structure of both semiconductor lasers, the thickness of the current limiting layer 6 is 4000Å in this embodiment.
However, the conventional example is different in that it is 10000 Å. The other film thicknesses are common to both, and the N-type cladding layer 3
Then 13000Å, active layer 4 is 500Å, lower clad layer 5 is 30
00Å, P-type upper clad layer 7 is 10000 Å, cap layer 8 is
It was 30,000 Å. In addition, a SAM (Self Aligned structure by MBE) structure that uses the MBE method for the crystal growth method (Haruo Tanaka, “Self-Aligned Structure Laser by MBE”)
(Japanese Journal of Applied Physics, 24
Volume, p. 89, 1985)).

The front end face (emission end face) of each semiconductor laser is coated with Al ₂ O ₃ in a thickness of λ / 4 (reflectance 10%.
The following), and Al ₂ O ₃ having a thickness of λ / 4, a-
Four layers of Si were coated with a thickness of λ / 4 (reflectance of about 90%).
From the comparison of FIG. 3, it can be seen that the semiconductor laser of the present invention has multiple longitudinal modes.

[0024]

As described above, in the semiconductor laser of the present invention, the thickness of the current limiting layer is 6000 to 2000Å, so that the following effects are obtained.

(1) A self-aligned high-power semiconductor that has a multi-longitudinal mode oscillation in a low-power operation (for example, an optical output of 3 mW) while having a refractive index guided structure capable of a single transverse mode up to a high-power operation. You can get a laser.

(2) Other characteristics (for example, low astigmatic difference) required for the magneto-optical disk described above can be conventionally satisfied.

(3) The production process is not complicated and the production yield is not reduced.

(4) Since the current limiting layer is thin, the time required for crystal growth can be shortened and the production efficiency can be improved.

[Brief description of drawings]

FIG. 1A is a cross-sectional explanatory view of an embodiment of a semiconductor laser of the present invention, and FIG. 1B is a band structure diagram of an MQW active layer.

FIG. 2 is a diagram showing changes in relative intensity noise when the thickness of a current limiting layer is changed.

3A is a diagram showing a longitudinal oscillation mode at an optical output of 3 mW of the semiconductor laser of the present embodiment shown in FIG. 1, and FIG.
FIG. 6 is a diagram showing a similar waveform obtained by a conventional semiconductor laser.

[Explanation of symbols]

 1, 9 electrode 2 substrate 3 clad layer 4 MQW active layer 5 lower clad layer 6 current limiting layer 7 upper clad layer 8 cap layer

Claims (1)

[Claims] 1. An AlGaA having a complex index guiding mechanism.
An s-based self-aligned MQW structure semiconductor laser, wherein the current limiting layer has a film thickness of 6000 to 2000Å.